Shanghai Institute of Optics and Fine Mechanics has made progress in the generation of third harmonic in laser air filamentation

Recently, the team from the State Key Laboratory of Intense Field Laser Physics, Shanghai Institute of Optics and Mechanics, Chinese Academy of Sciences found that the third-order harmonics induced by air filamentation of high repetition rate femtosecond lasers have significant self jitter. To solve this bottleneck problem, a solution based on an external DC electric field was proposed, which significantly reduced the beam direction and intensity jitter of third-order harmonics, providing an idea for generating stable, high-energy, high repetition rate ultraviolet and extreme ultraviolet laser sources. The related research results were published in Optics&Laser Technology under the title of Improving the Beam Pointing and Intensity Stability of the Third Harmonic Generation in Air Film.

Ultraviolet and extreme ultraviolet lasers have wide applications in high-resolution imaging, material processing, and advanced spectroscopy. Harmonics are one of the important means to obtain these short wavelength lasers. The traditional harmonic generation method based on nonlinear crystals is limited by the crystal's UV transmittance cutoff wavelength, damage threshold, and other factors, making it difficult to obtain high-energy UV light sources. The third harmonic induced by laser gas filamentation can overcome the above limitations.

In this work, the team studied the stability of the third harmonic induced by femtosecond laser air filamentation and found that under high repetition rate conditions, the jitter of the third harmonic beam generated by laser filamentation was significantly enhanced (Figure 1), which seriously affected the stability of the third harmonic and became a technical bottleneck restricting its application. To solve this problem, the research team innovatively introduced an external DC electric field, successfully suppressing the pointing and intensity jitter of the third harmonic beam. The experimental results showed that at a repetition rate of 1 kHz, the beam pointing jitter was reduced by about 50%, and the intensity jitter was effectively suppressed (Figure 2). This is because the external electric field can suppress the plasma recombination effect, reduce the heat deposition caused by plasma recombination between pulses, and thus reduce the intensity of airflow turbulence caused by thermal diffusion. The numerical simulation results of fluid dynamics further confirm the improvement of the stability of the laser filament plasma channel by the external DC electric field, thereby enhancing the stability of the third harmonic. This study not only lays the foundation for the development of high-energy ultraviolet and extreme ultraviolet laser sources with high repetition rate and high stability, but also opens up new opportunities for their applications in imaging, precision machining and other fields.

Figure 1. Cloud map of the pointing distribution of the third harmonic (TH) beam at different repetition rates. (a) 50Hz, (b) 100Hz, (c) 500Hz, (d) 1kHz. The variation curve of the pointing deviation distance (e) and intensity jitter (f) of the third harmonic beam with the repetition frequency of the filament. The red dots indicate the directional distribution of the pump laser before it becomes filamentous.

Figure 2. (a) Distribution cloud map of TH beam direction generated based on 1kHz optical fiber without and after applying an electric field. (b) The relationship between the deviation distance of 1kHz TH beam pointing and the voltage applied to the optical fiber. (c) The relationship between the intensity jitter of 1kHz TH and the voltage applied to the optical fiber.

This work has been supported by the joint fund project of the National Natural Science Foundation of China, the Shanghai Science and Technology Project, the key international cooperation project of the Chinese Academy of Sciences and other projects.

Source: Opticsky